EL84s are heating up and distorting

Steve Waltz · From: USA

This isn't guitar oriented but I thought I could get some help here. I wanted to try a tube Hi Fi amp at home for kicks. I found a 50's Bogen DB115. It's a 15 watt mono tube amp with two EL84 power tubes. I found it at an estate sale a few blocks away. It had a hum and no sound on arrival. I replaced the muti-section capacitor since it looked like it leaked. I also replaced the other two large electrolytic caps. Fired it up and it works and sounds pretty good other than the hum. The hum is there regardless of volume and doesn't get louder as volume goes up it just changes tone wise. I checked the tubes on an old tube tester and the EL84s were low. I bought a matched set of GT EL84 and put them in. The hum was almost gone but now it distorts big time and they were heating up way more than the old mullards. I put the old Mullards back and it works but has the hum.

There is a hum balance and a damping adjust but no bias adjust. I'm assuming it needs a bias adjust to fix the distortion but is it possible the new tubes are bad? I was just surprised by the dramatic difference by just a tube change out.Could the tubes be that different? It isn't worth much so I'm trying to get it going on my own.

steve

Jussi Huhtakangas · From: Helsinki, Finland

Steve, I'd try out yet another pair of tubes. I'd be surprised if a hi-fi amp needed a bias adjusting, at least mine doesn't and it's a Leak equivalent of yours, although I have separate pre and power amps ( I assume Bogen is an intergrated amp? ). Does yours look like this? :
http://estoreoutwest.com/product_info.php?products_id=55
I've found the best new tubes on the market to be JJ's, not sure how easily they're available over there though.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Sounds like one of the tubes has an inter-electrode short. That leaks the AC from the filament supply into the signal path, which would explain the hum. It also throws the bias of that tube way off, which would explain the distortion.

Steve Waltz · From: USA

Thanks for the input.

My amp is like Jussi's link but the faceplate is slightly different, but it's the same amp.I also got a Stephens Tru-sonic 120 and the Bogen R660 tuner with it. Not bad for $20. The speaker is perfect and the tuner works well.

I don't have any other amps that use EL84's so I don't have any to replace these with.

I've heard that the Groove tube EL84s are JJ's but Groove tube tests them and puts a rating on them. Mine have "5" which means they will have a mid level break up. The s after El84 stands for Slovakia.

Leo's pro audio will not take them back or exchange them since they are "tubes". Nice customer support!

It's sounding like the tubes are the issue so I guess I 'll have to go through GT for a replacement set. At least their customer support said they would help out.

Thanks for letting me know that the bias was most like likely not the issue.

Steve

Jim Sliff · From: Lawndale California, USA

Most E84-based amps are cathode biased, so your "bias adjustment" would involve checking current/voltages and replacing parts to get it in range. You won't install a "bias pot" like you would on a 6L6 or 6V6-powered amp.

But there's something else going on here - bias isn't going to cause hum, a faulty ground, ground loop or external source is - unless one of the tubes if faulty as Donny explained.

You "old tube tester" probably is useful as a boat anchor. Tube testers HAVE to be calibrated periodically, and are lousy for testing power tubes - most only hit the tubes with 160-185 volts on the plates, and a guitar (or hi fi) amp is going to probably have double to triple that voltage...all you can do really is check good/bad and get a VERY rough idea of matching, but you'd be lucky even with a calibrated tester if matched readings correlated to matched bias in an amp. These testers were made when tubes were consistent...which is no longer the case.

The combination of hum and distortion when tubes were changed have me thinking there may be a transformer issue and your voltages are WAY off. You should be able to dig up a schematic and check the plate voltages...also check all the other caps for leakage, and resistors - ones that look cooked probably are, and I'd bet more than a few have drifted way off-spec.

Old amps like this are fun but take a lot of know-how and work to tune up.
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Michael Brebes · From: Northridge CA

I agree with the comment that the EL84's are usually cathode biased. That means that there is a resistor from the two cathodes to ground. There should be an electrolytic capacitor across that resistor. You might want to try changing that as well.
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Michael Brebes
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Steve Waltz · From: USA

Thanks for the advice jim. The tube tester is a large comercial thing at Al lashers in Berkeley but i think you may be right since I tried a new set on that tester and it showed only a slightly beeter reading. But it still seems strange that the distortion happens only with the new tubes. The hum is also almost gone with the new tubes. If there was a transformer problem wouldn't the distrotion be there with the old and new tubes, which is not the case.

I did a quick check of many of the resistors by using a mutimeter and seeing how far off of the color code they were and most seemed fine. I replaced all of the electrolytic caps. I didn't touch the other coupling caps because I thought those were not prone to problems just like the blue caps in Fenders.
Bogen does use some of those mounting posts like valco and airline and I think those can have bad connections so I'll check those too for the hum. I was thinking it was most likely the EZ81 tube since it's as old as the original EL84's.

I think I'll just have to bit the bullet and buy another set of El84's since I still can't get teh store to take them back.

Larry Bressington · From: Nebraska

Does the hum change with volume, if it does not, then it a/c hum, if it does then its interference hum ala coax leakage, also you said that this is a hifi amp, i beleive the input impedance is very different to a pick-up coil. The mullard valves were awsome valves back then, try tapping gently on the valve with you finger nail and see what noise comes through the speaker, if its a good valve with no leakage, you will not hear a scrathy sound, if the valve is leaky it will be noisy. Very Happy

Lee Jeffriess · From: Vallejo California

Steve, I would take it to Skip Simmons, he is in Davis.
These amps are his Forte. You can find him on the web or E me and I will dig up his #.
Lee

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